1. Field of the Invention
This invention relates to wireless communications and, more particularly, to estimating carrier-to-interference ratio.
2. Description of the Related Art
Wireless communication devices such as mobile telephones, for example, transmit and receive signals including speech audio. Thus, they typically include a voice or speech encoder/decoder or “vocoder.” The vocoder may be used for compression/decompression of digital voice audio using audio compression algorithms. In addition, a channel encoder/decoder or channel codec may provide error protection of the received signal against channel imperfections by adding error detection/correction codes (EDC/ECC) in the transmitted signal. One type of vocoder uses a compression technology that is referred to as Adaptive Multi-Rate (AMR). In a system that supports AMR, the vocoder may dynamically adapt the audio compression bit rate dependent upon network channel conditions. More particularly, depending on the interference (and thus the signal quality) in the traffic channel, the vocoder bit rate may be increased or decreased to accommodate fewer or more EDC/ECC bits, for example.
A key attribute of such systems is the ability to rapidly adapt the rate using in-band signaling. One type of in-band signaling is referred to as codec mode request (CMR). CMR may be used by a mobile handset to request that the network use a specified downlink rate. The network may provide one or more signal quality thresholds to the mobile handset. The signal quality threshold values may correspond to carrier-to-interference (C/I) ratio values. During operation, the mobile handset may calculate C/I ratio estimates and compare them to the network-provided thresholds to determine which vocoder rate to request. Accordingly, the accuracy of the C/I ratio estimate affects the AMR performance. However, certain transmission modes such as discontinuous transmission (DTX) may prevent conventional C/I ratio estimation techniques from accurately estimating the C/I ratio.